实验变形岩石低温破裂作用的微观机制

通过对湿度和压力分别达300℃和3GPa干条件下实验变形的一系列花岗岩样品的系统光学和扫描电镜显微构造分析,识别出了几种具有不同特点及不同变形机制的显微破裂型式:a.微破碎带;b.微透镜带;c.短程破裂;d.扭折带破裂;e.粒内破裂;f.颗粒边界破裂。评价了它们与高压型和低压型破裂的显微构造对应性,认为a和f是低压破裂的特征性构造型式;b、c和d在高压破裂中最为典型;而e则可以出现在两种破裂型式中。本文阐述了破裂作用的微观机制,包括破裂的成核、扩展、生长和连接。破裂作用、由破裂诱发的脆性破裂和碎裂流动在目前实验变形条件下占主导地位,而在相对较高温条件下晶质塑性也起着一定的作用。破裂作用的成核可以始于颗粒边界,而引起摩擦颗粒边界滑移,也可以始于相邻颗粒或晶内包体颗粒的一些端点,或始于晶内双晶和解理面。破裂的生长和扩展一般是由于一些破裂的侧向延伸、相近破裂系统的联合和不同破裂系统的转变。低压样品的最终破裂一般归咎于一些贯穿性微断层的出现及主断层附近微破裂群的产生,微破碎角砾岩是其主要构造岩类型,类似于天然张性断层带构造;而沿主高压破裂附近却从未见到这种微破裂群的出现,主要表现为主破裂面及一些均匀分布的短程破裂构造, 典型构造岩为构造透镜体, 类似于天然压性与压扭性断层构造。两种破裂在成核、扩展与位移发育诸方面表现出的差别有助理解它们在声发射方面的差异。

MICROMECHANISMS OF LOW TEMPERATURE FRACTURING IN EXPERIMENTALLY DEFORMED CRUSTAL ROCKS

Systematic microstructural analyses have been done on a series of granite samples deformed under dry conditions up to T-300℃ and P-3GPa. Several styles of microfractures with different characteristics, and therefore deformation mechanisms, are recognized from their optical and SEM characteristics: a. micro-crash zones, b. transgranular lens zones, c. short length fractures, d. kink band fractures, e. intragranular fractures, and f. grain boundary fractures. Their microstructural correlation to the high-P (pressure) and low-P (pressure) type fractures is evaluated, where the low P fractures are generally characterized by a a nd f, while b and c are typical of high-P fractures. Fractures d may occur in both cases. These fracture styles contribute to the major parts of strains in the samples. The micromechanisms of fracturing is elucidated, including the nucleation, propagation, growth and connection of the fractures. Fracturing, fracturing-induced b rittle faulting and cataclastic flow dominate the deformation in the whole range of the experimental conditions, while crystalline plasticity may become secondary to fracturing at higher temperatures. Fracturing processes begin with fracture nucleation at grain boundaries inducing grain boundary sliding, at the tip points of secondary phase inclusions or the neighboring grains, and along twin or c leavage planes. Their growth and propagation is generally due to the extension of particular fractures, the coalescence of similar fracture systems or transform ation between different fracture systems. The final failure of low-P specimens is generally attributed to the occurrences of some through going faults together with further formation of fracture swarms along the main fault, while these swarms of secondary fractures have never been observed along the high-P fractures. Such a difference may also help understanding their differences in AE activity patterns.